To clarify, their stated payment terms are the last day of the month following the shop. Their emails stating checks have been mailed usually comes between the 10th and 12th with checks arriving around the 15th.
Are you saying you are getting emails around the 1st and checks a few days later?
My guess is that you are actually saying that their late payment does not bother you, but those are two entirely different things.
There are reasons that a body stays in motion
At the moment only demons come to mind